IX 1.2

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;

static class Program {
  static void Main() {
    var xs = EnumerableEx.Create<int>(async yield => {
      try {
        await yield.Return(42);
      } finally {
        Console.WriteLine("finally");
      }
    });

    var ys = Iterator();

    // prints: "xs: 42"
    foreach (var i in xs.Take(1))
      Console.WriteLine("xs: {0}", i);

    // prints: "xs: 42
    //          finally"
    foreach (var i in ys.Take(1))
      Console.WriteLine("ys: {0}", i);
  }

  private static IEnumerable<int> Iterator() {
    try {
      yield return 42;
    } finally {
      Console.WriteLine("finally");
    }
  }
}

For async methods C# compiler do not emits 'enumerator disposed early' control flow edges like in C# 2.0 iterators.
This is why any async-based anonymous iterators implementation can't correctly deal with .Take() or break; in foreach loops.

Great catch. You could use an explicit yield.Break(). Just to be very clear. This is really a hack to work around limitations in the C# language. So I am totally fine things are rough on the edges. Handle with care.